Skip to content

format safety doc of Rc/Arc::from_raw/from_raw_in #154081

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
hxuhack wants to merge 1 commit into
base: main
Choose a base branch
from
Open

format safety doc of Rc/Arc::from_raw/from_raw_in #154081

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
56 changes: 31 additions & 25 deletions library/alloc/src/rc.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1430,29 +1430,32 @@ impl<T: ?Sized> Rc<T> {
/// Constructs an `Rc<T>` from a raw pointer.
///
/// The raw pointer must have been previously returned by a call to
/// [`Rc<U>::into_raw`][into_raw] with the following requirements:
/// [`Rc<U>::into_raw`][into_raw] or [`Rc<U>::into_raw_with_allocator`][into_raw_with_allocator].
Copy link
Member

@hkBst hkBst Mar 19, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This information is now duplicated in the safety section, but there it is phrased negatively instead. Should perhaps this positive version be moved into the safety section instead?

Copy link
Contributor Author

@hxuhack hxuhack Mar 20, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I referred to the format used in Thread::from_raw, which follows the same pattern.

///
/// # Safety
///
/// * Creating a `Rc<T>` from a pointer other than one returned from
/// [`Rc<U>::into_raw`][into_raw] or [`Rc<U>::into_raw_with_allocator`][into_raw_with_allocator]
/// is undefined behavior.
Copy link
Member

@hkBst hkBst Mar 19, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I'm guessing it is also undefined behavior if the pointer came from a call to into_raw_with_allocator where the returned allocator was not the global one.

Copy link
Contributor Author

@hxuhack hxuhack Mar 20, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Yes, but there is another safety requirement in the safety section preventing this: the raw pointer must point to a block of memory allocated by the global allocator.

/// * If `U` is sized, it must have the same size and alignment as `T`. This
/// is trivially true if `U` is `T`.
/// * If `U` is unsized, its data pointer must have the same size and
/// alignment as `T`. This is trivially true if `Rc<U>` was constructed
/// through `Rc<T>` and then converted to `Rc<U>` through an [unsized
/// coercion].
///
/// Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
///
/// The raw pointer must point to a block of memory allocated by the global allocator
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
/// * Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
/// * The raw pointer must point to a block of memory allocated by the global allocator
/// * The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Rc<T>` is never accessed.
///
/// [into_raw]: Rc::into_raw
/// [into_raw_with_allocator]: Rc::into_raw_with_allocator
/// [transmute]: core::mem::transmute
/// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
///
Expand Down Expand Up @@ -1662,29 +1665,32 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
/// Constructs an `Rc<T, A>` from a raw pointer in the provided allocator.
///
/// The raw pointer must have been previously returned by a call to [`Rc<U,
/// A>::into_raw`][into_raw] with the following requirements:
/// A>::into_raw`][into_raw] or [`Rc<U, A>::into_raw_with_allocator`][into_raw_with_allocator].
///
/// # Safety
///
/// * Creating a `Rc<T, A>` from a pointer other than one returned from
/// [`Rc<U, A>::into_raw`][into_raw] or [`Rc<U, A>::into_raw_with_allocator`][into_raw_with_allocator]
/// is undefined behavior.
/// * If `U` is sized, it must have the same size and alignment as `T`. This
/// is trivially true if `U` is `T`.
/// * If `U` is unsized, its data pointer must have the same size and
/// alignment as `T`. This is trivially true if `Rc<U>` was constructed
/// through `Rc<T>` and then converted to `Rc<U>` through an [unsized
/// alignment as `T`. This is trivially true if `Rc<U, A>` was constructed
/// through `Rc<T, A>` and then converted to `Rc<U, A>` through an [unsized
/// coercion].
///
/// Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
///
/// The raw pointer must point to a block of memory allocated by `alloc`
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
/// * Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
/// * The raw pointer must point to a block of memory allocated by `alloc`
/// * The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Rc<T>` is never accessed.
/// even if the returned `Rc<T, A>` is never accessed.
///
/// [into_raw]: Rc::into_raw
/// [into_raw_with_allocator]: Rc::into_raw_with_allocator
/// [transmute]: core::mem::transmute
/// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
///
Expand Down
54 changes: 30 additions & 24 deletions library/alloc/src/sync.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1581,29 +1581,32 @@ impl<T: ?Sized> Arc<T> {
/// Constructs an `Arc<T>` from a raw pointer.
///
/// The raw pointer must have been previously returned by a call to
/// [`Arc<U>::into_raw`][into_raw] with the following requirements:
/// [`Arc<U>::into_raw`][into_raw] or [`Arc<U>::into_raw_with_allocator`][into_raw_with_allocator].
///
/// # Safety
///
/// * Creating a `Arc<T>` from a pointer other than one returned from
/// [`Arc<U>::into_raw`][into_raw] or [`Arc<U>::into_raw_with_allocator`][into_raw_with_allocator]
/// is undefined behavior.
/// * If `U` is sized, it must have the same size and alignment as `T`. This
/// is trivially true if `U` is `T`.
/// * If `U` is unsized, its data pointer must have the same size and
/// alignment as `T`. This is trivially true if `Arc<U>` was constructed
/// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
/// coercion].
///
/// Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
///
/// The raw pointer must point to a block of memory allocated by the global allocator.
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
/// * Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
/// * The raw pointer must point to a block of memory allocated by the global allocator.
/// * The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Arc<T>` is never accessed.
///
/// [into_raw]: Arc::into_raw
/// [into_raw_with_allocator]: Arc::into_raw_with_allocator
/// [transmute]: core::mem::transmute
/// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
///
Expand Down Expand Up @@ -1819,29 +1822,32 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
/// Constructs an `Arc<T, A>` from a raw pointer.
///
/// The raw pointer must have been previously returned by a call to [`Arc<U,
/// A>::into_raw`][into_raw] with the following requirements:
/// A>::into_raw`][into_raw] or [`Arc<U, A>::into_raw_with_allocator`][into_raw_with_allocator].
///
/// # Safety
///
/// * Creating a `Arc<T, A>` from a pointer other than one returned from
/// [`Arc<U, A>::into_raw`][into_raw] or [`Arc<U, A>::into_raw_with_allocator`][into_raw_with_allocator]
/// is undefined behavior.
/// * If `U` is sized, it must have the same size and alignment as `T`. This
/// is trivially true if `U` is `T`.
/// * If `U` is unsized, its data pointer must have the same size and
/// alignment as `T`. This is trivially true if `Arc<U>` was constructed
/// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
/// alignment as `T`. This is trivially true if `Arc<U, A>` was constructed
/// through `Arc<T, A>` and then converted to `Arc<U, A>` through an [unsized
/// coercion].
///
/// Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
///
/// The raw pointer must point to a block of memory allocated by `alloc`
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
/// * Note that if `U` or `U`'s data pointer is not `T` but has the same size
/// and alignment, this is basically like transmuting references of
/// different types. See [`mem::transmute`][transmute] for more information
/// on what restrictions apply in this case.
/// * The raw pointer must point to a block of memory allocated by `alloc`
/// * The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Arc<T>` is never accessed.
///
/// [into_raw]: Arc::into_raw
/// [into_raw_with_allocator]: Arc::into_raw_with_allocator
/// [transmute]: core::mem::transmute
/// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
///
Expand Down
Loading